The present invention relates to solid state relays and, more particularly, to solid state relays used in traffic signal light control systems.
Intersecting vehicle thoroughfares often have provided at their intersections standards supporting vehicle traffic control signal lights with one light of each color on each standard visible across the intersection from each of the thoroughfares. These signal lights are typically operated under the control of a control system, including the timing controller and the conflict monitor, housed in a control system cabinet provided in the vicinity of the intersection. The control system and the signal lights are typically operated using alternating current obtained from a commercial electrical power distribution system.
The green, red and yellow signal lights usually used at an intersection, under the direction of the control system, are typically operated to be continuously switched on for selected short time intervals, or are otherwise operated in a switched on-switched off, or flashing, sequence over other selected time intervals. The control system accomplishes such operation of the signal lights through generating signals in the timing controller to close and open load switches that control the supply of alternating currents through a flash transfer relay to the signal lights in those situations in which the signal lights are to be continuously switched on during selected intervals. The flash transfer relay is placed in a first switching state by the conflict monitor during times including those selected intervals that enables the load switches to be effective in selecting those intervals for delivery of alternating current to the lights. In those intervals in which the signal lights are to flash, the conflict monitor directs this condition through the flash transfer relay by changing the relay to another, or second, switching state.
The flash transfer relays provided in typical traffic signal light control systems have traditionally been electromagnetic devices using the presence or the absence of current in a coil to create or end magnetic fields to open or close electrical contacts. These contacts, in the absence of contamination, or other electrical conductivity limiting effects, tend to have very small voltage drops thereacross during times they are closed and carrying substantial electrical currents therethrough. Thus, they dissipate relatively little electrical power during the carrying of substantial electrical currents therethrough. On the other hand, such contacts suffer from impact recoil so that they rapidly open and close following the first making contact in a switch closing thereby leading to "chattering", and there are discharges between the contacts as they come close to one another leading to "arcing" which causes corrosion and erosion of those contacts. Such contacts, of course, are already subject to contamination from external sources if they are not in a sealed enclosure. In addition, such behavior often leads to the generation of unwanted electrical noise in the circuits connected thereto and in circuits located nearby.
Because of these deficiencies in electromechanical relays, solid state relays have been developed as substitutes in some situations. Such relays use power semiconductor devices which can operate as switches to switch on and off relatively large currents, and can with stand relatively large voltages thereover when switched off. The use of such semiconductor device switches avoids the presence of any moving parts which can wear out, and avoids the use of contacts which are subject to erosion, corrosion and contamination, and thereby also avoids contact chattering and the undue generation of electrical noise. However, such power semiconductor switches do have a relatively large voltage drop thereacross even when switched on to conduct current therethrough and, thus, these devices are subject to significant electrical energy dissipation.
The use of a solid state relay for flash transfer relays in traffic signal light control systems of typical design is difficult because of the electrical signals available to operate the flash transfer relay. In typical control systems of the past, electromechanical relays were used which merely need to have an alternating current supply provided to the coil thereof whenever relay contacts therein are to be actuated, and removed therefrom when those relay contacts are to no longer be actuated. Thus, such a selectively provided alternating current supply serves as the operating signal for switching the flash transfer relay between switching states thereof.
In some typical traffic signal light control systems operating normally, such signal operation control alternating current is always supplied when the signal lights are to be switched on continuously over selected intervals, and this alternating current is removed when the signal lights are to flash (an "energized"system). Other typical signal light control systems are operated in the opposite manner with the signal operation control alternating current being supplied only during instances in which the signal lights are to be flashing, and otherwise not provided when the signal lights are to be switched continuously on over selected intervals (a "deenergized" system).
However, these traffic signal light control systems, if operated using constant polarity voltages, can have large portions thereof operated with voltages of smaller magnitudes to thereby reduce risks to those who install and maintain such systems. In addition, such systems operated in large part with constant polarity voltages allow the use of less costly components as well as permitting reduced electrical power consumption, and often facilitate the provision of further circuit features. Furthermore, transients and electrical noise are often reduced on the power supply interconnections by the use of an alternating polarity to constant polarity voltage converter to provide the constant polarity operating voltage for portions of the system. Finally, the increasing use of light-emitting diode arrays as traffic intersection signal lights has led to a desire to operate them using constant polarity voltages in the future, and so is also leading to a desire to operate the control systems therefor using the same kinds of voltages even now with alternating polarity voltage operating these signal lights. Thus, there is a desire to have a solid state relay for the control system flash transfer relay operated from a constant polarity electrical energy supply.